1. Field
The present disclosed embodiments relate generally to measurement and control of fluid flow, and more specifically to model-based controller gain scheduling for mass flow controllers.
2. Background
A mass flow controller (MFC) is a device that sets, measures, and controls the rate and amount of fluid, such as a gas or vapor, passing through the device. These devices are designed and calibrated to control with high precision the flow of gas within a predefined range of flow rates.
Some manufacturing processes, such as semiconductor manufacturing, require precise control of the rate of flow and the amount (mass) of gases and vapors delivered to a process chamber or tool. Often, precise fluid flow rates and amounts (the total mass of fluid) are critical to ensure optimal results. Applications include metering of precise amounts of gases and vapors flowing into a process chamber for subsequent deposition on work pieces, dry etching to remove material, ion and plasma beams used in the semiconductor and pharmaceutical industries, etc. Mass flow controllers are particularly suited for these and other tasks.
Referring to FIG. 1, a mass flow controller system 10 typically couples a mass flow controller 12 with a flow channel 14. A select fluid enters the channel through an inlet 16, passes through the controller 12 and leaves the controller through an outlet 18 in a precisely controlled manner. The fluid flow path 20 splits inside the controller 12 into a bypass path and a sensor path. Specifically, at the first, divergence juncture 22, a fraction of the fluid splits off from the main path 20 and is divided between the main bypass path of a bypass tube 26, and a sensor path of the capillary sensor tube 24 of sensor 40. The sensor tube 24 and bypass tube 26 are rejoined at a second, convergence juncture 28 located downstream from the first juncture 22. This second junction 28 recombines the fluid flowing through the paths of the sensor and bypass tubes 24 and 26. The recombined fluid 30 then passes through a control valve 50, which in turn controls the fluid flow, as indicated at 36 as it exits the controller 12 through the outlet 18. A laminar flow or flow splitter element 32 is usually disposed in bypass tube 26 between the first and second junctures 22 and 28 so as to create laminar flow through the bypass tube 26 between the two junctures. As a result of laminar flow through the bypass tube and the internal capillary dimensions of the capillary tube, the mass flow through the capillary and bypass tubes will remain at a precise bypass ratio throughout the pre-designed rate of flow range of the instrument.
The sensor 40 is designed to provide a signal representative of the rate of flow of fluid flowing through the sensor tube 24. Since the bypass ratio remains fixed throughout the pre-designed rate of flow range of the instrument, the signal provided by the sensor 40 is representative of the combined flow through both the bypass and sensor tubes 24 and 26 (i.e., the combined flow through the mass flow controller system 10). The signal output of the sensor 40 is applied to the processor 60.
Sensors are known for measuring flow based on temperature measurements, while others are known for measuring flow based on pressure measurements. A typical thermal-based sensor has two sensor coils 44, 46, the upstream coil for injecting heat into the gas or vapor flowing through the capillary tube, and the other downstream for measuring the temperature loss between the two coils. This temperature difference is representative of the flow rate. Other thermal sensor arrangements are known, including sensors employing one and three coils.
The controller 12 also includes a control valve 50 used to control the fluid flowing response to a signal from the processor 60. The latter is configured and arranged so as to compare the actual flow as sensed by the sensor 40 to a set point (SP) value, usually determined by the user and/or the process being controlled, and provide a signal to the valve 50 so as to set the valve 50 at a correct position so as to produce the desired flow established by the SP value. Thus, if the processor 60 detects, from the sensors 40, that fluid is flowing too quickly, the processor 60 would send a signal to the control valve 50 to decrease the fluid flow rate, and vice versa.
The processor 60 may be configured so as to control the valve in at least two valve designs. One valve design includes a normally-open valve wherein the valve remains open until an electric signal is sent to the control valve. Another design is the normally-closed valve design wherein the valve is closed until an electric signal is sent to the control valve.
Mass flow controllers have also been designed to be insensitive to pressure fluctuations upstream from the controllers. An example of a pressure insensitive mass flow controller (πMFC or piMFC) is described in U.S. Pat. No. 6,712,084 issued to Ali Shajii, et al, and assigned to the present assignee. The patented πMFC also includes a thermal-based flow sensor, and is similar to the MFC described thus far in connection with FIG. 1, and further includes a pressure sensor 70 (shown by dotted lines in FIG. 1) coupled to measure the pressure of the fluid upstream from the valve 50. The pressure sensor 70 provides a signal to the processor 60 representative of the measured pressure of the fluid flowing through the device. Pressure insensitivity control is achieved in the patented πMFC through coordinated operations among the sensor 40, the processor 60, and the control valve 50, as well as using pressure measurements made by the pressure sensor 70. The goal of the patented πMFC is for the output flow 36 to be insensitive to upstream or downstream pressure disturbances. Algorithms (processed in the processor 60) are used to control the control valve 50 based on sensor inputs as well as the upstream pressure measurements. The πMFC further includes a temperature sensor 80 attached to the flow body for measuring the temperature of the fluid flowing into the controller.
In operation, the MFC controls the openness of the valve via feedback control loop such that the output flow rate, Qr, tracks the flow set point, SP. The controller gain of the MFC is usually determined at a known calibration condition, e.g. 40 psia inlet Nitrogen gas at room temperature, to have a good control performance when the flow set point changes. Two common control performance issues often occur for a MFC operating in a condition different from the calibration condition such as different gas type, different inlet gas pressure, or different inlet gas temperature. The first common control performance issue is overshoot when the set point changes as shown in FIG. 2A. This issue is related to a low controller gain setting when the inlet gas is switched from Nitrogen to a light weight gas such as Helium or the inlet gas pressure changes from high pressure to low pressure. The second common control performance issue is oscillation when the set point changes as shown in FIG. 2B. This issue is related to a high controller gain setting when the inlet gas is switched from Nitrogen to a heavy weight gas such as SF6 or the inlet gas pressure changes from low pressure to high pressure. These control issues are highly problematic in semiconductor manufacturing processes that require careful, high precision, time control of fluid flow rates
One approach to overcome this problem of differing responses depending on the pressure and type of gas flowing through an MFC is discussed in U.S. Pat. No. 6,962,164 issued to Lull. Lull tries to overcome the control issues by empirically determining a valve gain term based on a change of fluid flow divided by a corresponding change in valve displacement (of the valve used in the thermal-based MFC) at a plurality of predetermined flow rates. In other words, defining the gain setting as an operation-dependent function by correlating fluid flow with valve displacement. Using predetermined flow rates, Lull deduces the appropriate flow rate for a given condition (i.e., valve displacement) and adjusts the gain of the instrument appropriately. It is thought that by using data from different predetermined flow rates, the valve gain term would be more accurate. However, this approach is unnecessarily complicated.
There is therefore a need in the art for MFCs that can provide more reliable control of fluid flow in which the actual flow more smoothly settles on the set point without either overshoot or oscillation control responses.